"Our work cross-correlated two types of datasets from complementary, but very distinct, surveys," says Madhavacheril, "and what we found was that for the most part, the story of structure formation is remarkably consistent with the predictions from Einstein's gravity. We did see a hint for a small discrepancy in the amount of expected clumpiness in recent epochs, around four billion years ago, which could be interesting to pursue."
The data, published in the Journal of Cosmology and Astroparticle Physics and the preprint server arXiv, comes from the Atacama Cosmology Telescope's (ACT) final data release (DR6) and the Dark Energy Spectroscopic Instrument's (DESI) Year 1. Madhavacheril says that pairing this data allowed the team to layer cosmic time in a way that resembles stacking transparencies of ancient cosmic photographs over recent ones, giving a multidimensional perspective of the cosmos.
"ACT, covering approximately 23% of the sky, paints a picture of the universe's infancy by using a distant, faint light that's been travelling since the Big Bang," says first author of the paper Joshua Kim, a graduate researcher in the Madhavacheril Group. "Formally, this light is called the Cosmic Microwave Background (CMB), but we sometimes just call it the universe's baby picture because it's a snapshot of when it was around 380,000 years old."
The path of this ancient light throughout evolutionary time, or as the universe has aged, has not been a straight one, Kim explains. Gravitational forces from large, dense, heavy structures like galaxy clusters in the cosmos have been warping the CMB, sort of like how an image is distorted as it travels through a pair of spectacles. This "gravitational lensing effect," which was first predicted by Einstein more than 100 years ago, is how cosmologists make inferences about its properties like matter distribution and age.
DESI's data, on the other hand, provides a more recent record of the cosmos. Based in the Kitt Peak National Observatory in Arizona and operated by the Lawrence Berkeley National Laboratory, DESI is mapping the universe's three-dimensional structure by studying the distribution of millions of galaxies, particularly luminous red galaxies (LRGs). These galaxies act as cosmic landmarks, making it possible for scientists to trace how matter has spread out over billions of years.
"The LRGs from DESI are like a more recent picture of the universe, showing us how galaxies are distributed at varying distances," Kim says, likening the data to the universe's high school yearbook photo. "It's a powerful way to see how structures have evolved from the CMB map to where galaxies stand today.
By combining the lensing maps from ACT's CMB data with DESI's LRGs, the team created an unprecedented overlap between ancient and recent cosmic history, enabling them to compare early- and late-universe measurements directly. "This process is like a cosmic CT scan," says Madhavacheril, "where we can look through different slices of cosmic history and track how matter clumped together at different epochs. It gives us a direct look into how the gravitational influence of matter changed over billions of years."
In doing so they noticed a small discrepancy: the clumpiness, or density fluctuations, expected at later epochs didn't quite match predictions. Sigma 8 (s8), a metric that measures the amplitude of matter density fluctuations, is a key factor, Kim says, and lower values of s8 indicate less clumping than expected, which could mean that cosmic structures haven't evolved according to the predictions from early-universe models and suggest that the universe's structural growth may have slowed in ways current models don't fully explain.
This slight disagreement with expectations, he explains, "isn't strong enough to suggest new physics conclusively-it's still possible that this deviation is purely by chance."
If indeed the deviation is not by chance, some unaccounted-for physics could be at play, moderating how structures form and evolve over cosmic time. One hypothesis is that dark energy-the mysterious force thought to drive the universe's accelerating expansion-could be influencing cosmic structure formation more than previously understood.
Moving forward, the team will work with more powerful telescopes, like the upcoming Simons Observatory, which will refine these measurements with higher precision, enabling a clearer view of cosmic structures.
+ Mathew Madhavacheril is an assistant professor in the Department of Physics and Astronomy in the School of Arts and Sciences at the University of Pennsylvania.
+ Joshua Kim is a Ph.D. candidate at Penn Arts and Sciences.
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